1. Field of the Invention
This invention relates to the manufacture of composite load bearing structures, and in particular to the manufacture of an aircraft wing spoiler of honeycomb sandwich construction. The invention also relates to a design configuration for composite load bearing structures in general and in particular to a design configuration for an aircraft wing spoiler of honeycomb sandwich construction.
2. Description of Related Art
Because of the weight savings offered by advanced composite materials, especially epoxy graphite, and the resulting improvements in performance, modern aircraft have become increasingly reliant on such materials. The F-16 was the first military aircraft to use graphite epoxy structures in production, and the technology has advanced to the point where many aircraft, such as the F-18, use graphite composites extensively, in applications ranging up to 26% of the aircraft's structural weight. Future military aircraft designs are projected to include as much as 40-50% of their structural weight in advanced composites.
The commercial and general aviation industries have also utilized advanced composite technology, in conventional transport aircraft, the resulting weight savings providing significant increases in payload and consequent decreases in fuel consumption. Advanced composites have already been certified by the FAA for secondary structural (not critical to flight safety) components, and transport aircraft have been designed with advanced composite secondary structures including fairings, control surfaces, and the like.
A disadvantage of the graphite epoxy control surface, however, is that the cost to weight ratio is relatively high in comparison with metals such as aluminum. The reason for the high cost is that structural requirements for graphite epoxy control surfaces currently necessitate a concentration of graphite material piles and, in many configurations, the use of multiple pre-cured parts in order to provide spars and other structural elements necessary to accommodate fitting attachments. Construction of control surfaces having a large number of graphite material piles and pre-cured parts requires multiple curing cycles in the autoclave, greatly increasing manufacturing time and costs.
FIG. 1 is an exploded view of a current graphite epoxy spoiler design for a transport aircraft. The spoiler 10 is a special form of control surface which is located on the upper surface of the trailing edge of the wing 11 as shown in FIG. 3a, and which deflects upward from flaps 12 under pilot command in order to provide roll control and braking functions. The principal load on the spoiler when the spoiler is extended is therefore a shear force resulting from the airstream at the top of the wing which is normal to the spoiler when the spoiler is in its operative position.
The current design utilizes a honeycomb sandwich core construction, and requires six pieces, not including the three attachment fittings. The six pieces include respective pre-cured upper and lower covers 1 and 2, a pre-cured spar 3 for providing shear attachment of mounting fittings 4-6 to the control surface, and pre-cured closure ribs 7 and 8 which enclose the honeycomb core 9. In order to construct this type of spoiler, the various parts are separately pre-cured and then bonded together to form the spoiler assembly. The mounting fittings 4-6 are then attached to spar 3. The mounting hardware defines three hinge apertures 11-13 and an aperture 14 for attaching an actuator control rod.
This design, while structurally and aerodynamically acceptable, suffers from high manufacturing costs due to the relatively large number of graphite plies and curing steps to form the final assembly.